1. Field of the Invention
This invention is directed to a coating composition, in particular, to a coating composition useful as a primer, a primer surfacer or a primer filler having excellent chip resistance and good sandability.
2. Description of the Prior Art
To meet the many requirements of an exterior finish for automobiles, the automotive industry currently is using multi-layer finishes on automobile and truck bodies and parts. Typical of these finishes are layers of the following: (1) an electrocoat layer applied over a substrate, typically, a phosphatized cold rolled steel, (2) a primer layer, (3) a colored layer, typically pigmented, and (4) a clear layer. A colored top-coat layer may be used in place of the colored layer and clear layer. On repairing or refinishing such multi-layer finishes, a suitable primer, primer surfacer or primer filler coating is applied over the multi-layer finish that usually is sanded thereby exposing one or more layers or is applied over a filler material that has been used to fill in surface imperfections. This primer, primer surfacer or primer filler, herein after, “primer”, has many requirements. It must have adhesion to the substrate and provide a surface to which the colored layer or top-coat will adhere. It must be readily sandable in a reasonably short period of time after application, for example, about three hours after application. It must provide the resulting multi-layer finish with good impact resistance, in particular, stone chip resistance.
For a primer to exhibit the above properties, the cured primer layer should have high flexibility at low temperatures combined with high hardness under low stress conditions. In general, soft primers that have a high degree of flexibility and chip resistance, have poor sandability due to their rubber like nature and hence, cannot be processed rapidly in an auto refinish operation, which results in lowered productivity. If a primer is used that can be readily sanded after a very short drying time, it generally is hard and has poor flexibility and chip resistance. That problem has been partially overcome by using activated base coats to improve the chip resistance of the resulting multi-layer finish. However, this further complicates the refinishing process with the use of another reactive coating that has a limited “pot life”.
There are several primer compositions that can be used. For example, Barsotti, et al. U.S. Pat. No. 6,221,494, teaches the use of a solvent based ambient temperature curable high solids urethane coating useful for refinishing automobile and truck bodies and parts. Harris, et al. U.S. Pat. No. 5,596,043 shows a powder coating composition containing urethane and acrylic resins useful for coating that have flexibility and stone chipping resistance and McNeil, et al. U.S. Pat. No. 6,210,758 also shows a coating composition of an acrylic polymer, a polyurethane and a crosslinking agent that has improved chip resistance.
It would be very desirable to have a liquid coating composition, in particular, a primer composition, that cures at low temperatures, preferably, ambient temperatures and provides a combination of early hardness with good sandability that in combination with a base-coat and clear-coat forms a multi-layer finish on an automobile or truck body or part that has good stone chip resistance. More particularly, it would be desirable that such a composition contains components that are derived from renewable resources. The novel composition of this invention meets these aforementioned requirements.